214 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



been analyzed and found to be the relatively rare mineral dela- 

 fossite. 22 This occurrence will be further described by Mr. Ross in 

 a report which is in preparation. 



THE CALCITE GROUP 



The calcite group embraces the rhombohedral carbonates of 

 which the simple end members known from Idaho are: 



Calcite Calcium carbonate CaCO-3 



Siderite Iron carbonate FcCOj 



Rhodochrosite Manganese carbonate MnCO 



Smithsonite Zinc carbonate ZnCO a 



In addition to these simple carbonates there are some double carbon- 

 ates recognized, the most important being: 



Dolomite Calcium-magnesium carbonate CaC0 3 . MgCO-3. 



Ankerite Calcium-iron carbonate CaC0 3 .FeCOa 



While the above formulas represent the end compounds, the 

 natural minerals vary in composition by solid solution of the end 

 members in each other. In the simple carbonates the miscibility 

 seems limited. Siderite and rhodochrosite, of the simple carbonates, 

 seem liable to variation in composition, but calcite is much less 

 variable. There appears to be a complete gradation in the double 

 carbonates between dolomite and ankerite. The minerals of this 

 carbonate group do not occur in Idaho in good crystallizations nor 

 as unusually fine specimens, but siderite and ankerite are widely 

 distributed as gangue minerals in important ores and there seems to 

 be some definite relationship between the composition of the gangue 

 carbonate and the nature of the valuable minerals of the ore. For 

 this reason the data in regard to the members of this group at hand 

 are presented in some detail. Enough time has not been available 

 to permit the making of a sufficient number of analyses, in advance 

 of the preparation of this section, to permit of very definite con- 

 clusions, but it is hoped that the subject may be given more attention 



at a later date. 



CALCITE (270) 

 Calcium carbonate, CaCO-3 Rhombohedral. 



No localities furnishing unusually large or fine crystallizations of 

 calcite are known in Idaho, the mineral occurring most commonly 

 as a gangue mineral in ore-bearing veins or as limestone and marble. 

 The localities noted by the writer are in part as follows: 



22 Clyde P. Ross. A new copper district near Salmon, Idaho. Eng. and Min. Jour., vol. 118, No. 6, 

 Aug. 9, 1924, pp. 205-208, 1924. 



